The select example

select

Selection of scene objects using select() and an GL_SELECT render mode.

Use the select() callback function to implement your object selection function. This examples is based on a generic GL_SELECT implementation that can easily be cut and pasted in your applications.

Analytic intersection computations are also possible once the screen coordinates have be converted to a half line using convertClickToLine(). Make a selection and then move the camera to see a representation of the intersection line.

select.h

#include <QGLViewer/qglviewer.h>

class Viewer : public QGLViewer
{
protected :
  virtual void draw();
  virtual void drawWithNames();
  virtual void postSelection(const QPoint& point);
  virtual void init();
  virtual QString helpString() const;

private :
  qglviewer::Vec orig, dir, selectedPoint;
};

select.cpp

#include "select.h"
#include <qmessagebox.h>
#include <math.h>

using namespace std;

static void drawSpiral(const bool specialColor = false)
{
  const float nbSteps = 100.0;
  glBegin(GL_QUAD_STRIP);
  for (float i=0; i<nbSteps; ++i)
    {
      float ratio = i/nbSteps;
      float angle = 21.0*ratio;
      float c = cos(angle);
      float s = sin(angle);
      float r1 = 0.5 - 0.3*ratio;
      float r2 = 0.3 - 0.3*ratio;
      float alt = ratio - 0.5;
      const float nor = .5;
      const float up = sqrt(1.0-nor*nor);
      if (specialColor)
	glColor3f(1.0-ratio, 0.8f , ratio/2.0);
      else
	glColor3f(1.0-ratio, 0.2f, ratio);
      glNormal3f(nor*c, nor*s, up);
      glVertex3f(r2*c, r2*s, alt+0.05f);
      glVertex3f(r1*c, r1*s, alt);
    }
  glEnd();
}

void Viewer::drawWithNames()
{
  // Draw spirals, pushing a name (id) for each of them
  const int nb = 10;
  for (int i=0; i<nb; ++i)
    {
      glPushMatrix();
      glTranslatef(cos(2.0*i*M_PI/nb), sin(2.0*i*M_PI/nb), 0.);

      glPushName(i);
      drawSpiral();
      glPopName();

      glPopMatrix();
    }
}

void Viewer::postSelection(const QPoint& point)
{
  // Compute orig and dir, used to draw a representation of the intersecting line
  camera()->convertClickToLine(point, orig, dir);

  // Find the selectedPoint coordinates, using camera()->pointUnderPixel().
  bool found;
  selectedPoint = camera()->pointUnderPixel(point, found);
  selectedPoint -= 0.01f*dir; // Small offset to make point clearly visible.
  // Note that "found" is different from (selectedObjectId()>=0) because of the size of the select region.

  if (selectedName() == -1)
    QMessageBox::information(this, "No selection",
			     "No object selected under pixel " + QString::number(point.x()) + "," + QString::number(point.y()));
  else
    QMessageBox::information(this, "Selection",
			     "Spiral number " + QString::number(selectedName()) + " selected under pixel " +
			     QString::number(point.x()) + "," + QString::number(point.y()));
}

void Viewer::init()
{
  restoreStateFromFile();

  glLineWidth(3.0);
  glPointSize(10.0);

  help();
}

void Viewer::draw()
{
  // Draw ten spirals
  const int nb = 10;
  for (int i=0; i<nb; ++i)
    {
      glPushMatrix();
      glTranslatef(cos(2.0*i*M_PI/nb), sin(2.0*i*M_PI/nb), 0.0);
      drawSpiral(i == selectedName());
      glPopMatrix();
    }

  // Draw the intersection line
  glBegin(GL_LINES);
  glVertex3fv(orig);
  glVertex3fv(orig + 100.0*dir);
  glEnd();

  // Draw (approximated) intersection point on selected object
  if (selectedName() >= 0)
    {
      glColor3f(0.9f, 0.2f, 0.1f);
      glBegin(GL_POINTS);
      glVertex3fv(selectedPoint);
      glEnd();
    }
}

QString Viewer::helpString() const
{
  QString text("<h2>S e l e c t</h2>");
  text += "Left click while pressing the <b>Shift</b> key to select an object of the scene.<br><br>";
  text += "A line is drawn between the selected point and the camera selection position. ";
  text += "using <i>convertClickToLine()</i>, a useful function for analytical intersections.<br><br>";
  text += "To add object selection in your viewer, all you need to do is to define the <i>drawWithNames</i> function. ";
  text += "It gives a name to each selectable object and selection is then performed using the OpenGL <i>GL_SELECT</i> render mode.<br><br>";
  text += "Feel free to cut and paste this implementation in your own applications.";
  return text;
}

main.cpp

#include "select.h"
#include <qapplication.h>

int main(int argc, char** argv)
{
  QApplication application(argc,argv);

  Viewer viewer;

  viewer.setWindowTitle("select");

  viewer.show();

  return application.exec();
}

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